1. Field
The present invention relates to an image processing apparatus, an image processing method.
2. Description of the Related Art
In the advancing information society, information leakage is a serious problem, and there is a need of developing a technology for preventing the information leakage. For example, there is a developed technology of encrypting digital data so that a third person cannot read the data even if the data is stolen, which has already come into use as an effective way to prevent the information leakage.
On the other hand, there is not yet an effective technology for preventing the information leakage from printed materials, such as a paper medium. Therefore, there is a demand for immediate development of a technology for preventing the information leakage from the printed materials, like the digital data.
Examples of the printed materials that need to be protected from the information leakage include a purchase receipt, a personal ID such as a credit card number and a social security number, a medical record, a report card from school, and a customer list. The present invention can, for example, prevent the information leakage from them by encrypting a significant part of the information.
Conventionally known technologies of encrypting and decrypting printed data include a method of sectioning a document image into a plurality of blocks and rearranging the blocks (see, for example, U.S. Pat. No. 4,459,611), a method of printing an encryption rule in the form of a bar-code on an encrypted document (see, for example, Japanese Patent No. 2963472, pp. 1-4, FIG. 1), a method of black-and-white reversing or mirror reversing each of the rearranged blocks (see, for example, Japanese Patent Application Laid-open No. H8-179689, p. 4, FIG. 3), a method of rotating each of the rearranged blocks (see, for example, Japanese Patent No. 3609097, p. 4, FIG. 3), and a method of adding reference marks to an encrypted image for position detection and detecting borders of the blocks based on the reference marks for performing at least one of recording, scaling, rotating, shifting, and covering of a deficiency, at the time of decryption (see, for example, Japanese Patent Application Laid-open No. H9-504660, p. 13, FIG. 2).
To summarize the method of sectioning an image into a plurality of blocks and rearranging the blocks as described above (hereinafter, “scrambling technique”), when an image processing apparatus performs encryption, the image processing apparatus sections an input image, or an area of the input image to be encrypted, into a plurality of blocks, rearranges or scrambles the blocks based on parameters obtained from an encryption key input to the image processing apparatus, and converts pixel values using such a technique as black-and-white reversing, so that the position of each block can be detected.
To decrypt the image encrypted by the scrambling technique, the image processing apparatus detects the encrypted area and blocks in the encrypted area, and rearranges or reverse-scrambles the blocks based on the parameters obtained from a decryption key input to the image processing apparatus.
However, the above conventional technologies involve a risk that, as described below, the image processing apparatus cannot form a converted image from which a correct decryption can be performed even if the image is degraded, where the converted image means an image that has undergone the scrambling process and the conversion of the pixel values.
For example, with the technologies described in U.S. Pat. No. 4,459,611, Japanese Patent No. 2963472, and Japanese Patent No. 3609097, the position of each block cannot be corrected precisely, and therefore the decryption cannot be performed correctly. More specifically, the conversion of the pixel values is not performed in these technologies, and therefore the position of each block cannot be corrected precisely if the image is distorted by printing or scanning.
Furthermore, with the technologies described in U.S. Pat. No. 4,459,611, Japanese Patent Application Laid-open No. H8-179689, and Japanese Patent Application Laid-open No. H9-504660, by reversing the whole block or by adding the reference marks to the blocks, black pixels increase after the encryption, which cause blurs in printing the image. In this manner, the converted image formed by these technologies cannot correctly reproduce the image.
Moreover, for example, if the encrypted image is in the form of digital data, the information can be completely reproduced when the blocks are black-and-white reversed. However, if the encrypted image is printed and scanned by a scanner, an image of the black-and-white-reversed area cannot be completely reproduced.
Furthermore, for example, because the technologies described in Japanese Patent Application Laid-open No. H8-179689 and Japanese Patent Application Laid-open No. H9-504660 employ black-and-white reversing to convert pixel values, these technologies cannot be applied to an image having multiple tones.
More specifically, with these conventional technologies, the image processing apparatus detects a block by detecting an edge formed on the border between a reversed area and a non-reversed area. This technique postulates that the information to be encrypted is basically black-and-white characters or a black-and-white drawing, and it cannot be applied to an image having the multiple tones, such as a photograph. For example, if the technique is applied to an image with a half tone background as shown in FIG. 23A, the pixel values of the background hardly change as shown in FIG. 23B, and therefore it is hard to detect the border of the blocks.